Known systems employ ECG-gated (Turbo) spin echo (TSE) imaging for MR Angiography using an intrinsic contrast mechanism by exploiting differences in the velocity of arterial blood during a cardiac cycle. In this type of imaging, an MR signal from stationary tissue and non-pulsatile venous blood is canceled by image subtraction, while an MR signal from arterial blood is preserved. This is because, faster flowing blood has a low MR signal on TSE images due to de-phasing effects, while slow flowing blood has a higher signal. Known systems acquire two datasets during diastolic and systolic cardiac phase as illustrated in FIG. 1 corresponding to fast and slow flow periods of the cardiac cycle using ECG triggering. The datasets are subtracted and post processed using maximum intensity projection (MIP). The known systems employ readout methods including SPACE based readout (using a variable flip angle method) and Turbo Spin Echo readout including HASTE (Half Fourier Acquisition STE) as identified in FIG. 2.
The readout direction of known systems for limb imaging, for example, is along the direction of blood flow i.e. longitudinally along a limb as illustrated in FIG. 3 because this is the longest dimension of the anatomy and ensures efficient data acquisition. In order to obtain adequate recovery of magnetization between successive RF (radio frequency) excitation pulses, data is typically collected in one or more cardiac cycles. Many k-space lines (typically k-space lines of an entire slice (partition)) need to be collected per echo train to image a 3D volume within a reasonable scan time as illustrated in FIG. 4. Specifically, FIG. 4 illustrates acquisition of a slice every two heart cycles. Further, in order to achieve a larger field of view (FOV) coverage, known MR systems typically use a coronal orientation together with a localization scan to find vessels prior to an MRA scan which is time-consuming and operator dependent.
In known MR imaging systems, an entire 3D volume is excited with RF excitation fields over multiple cardiac cycles and typically two 3D datasets are acquired with one being acquired during a fast blood flow (e.g. systolic) cardiac period and another being acquired during a slow blood flow (diastolic) cardiac period. For example, known system perform imaging using a single shot SPACE or TSE sequence, with 80 partitions (slices) for 3D volume imaging, triggered by an RR waveform so that 160 RRs are needed, which is time consuming and therefore sensitive to respiratory motion disturbance. A system according to invention principles addresses the deficiencies of known MR imaging systems and related problems.